Test Gas Concentrations used under BS EN 14181

Importance of using suitable test gas concentrations for compliant emission monitoring

Measuring and monitoring emissions is fundamental to understanding the level of pollutants entering the air, assessing the performance of control measures and ensuring compliance with regulations.

Under BS EN 14181 Stationary Source Emissions: Quality Assurance of automated measuring systems, there are three specified quality assurance levels (QALs) that must be adhered to in order to be compliant. The suitability of the Continuous Emissions Monitoring System (CEMS) can be established by following the three QALs, which strategically refer to the installation, calibration and operation of the CEM.

To complete the checks, there are a number of different cylinder test gases used on operator sites. Here, Andy Tiffen, project manager for the Air & Emissions division of SOCOTEC explains the purpose and requirements for the different uses.

Compliant emission monitoring

Complying with BS EN 14181 is a requirement for operating under Directive 2010/75/EU on industrial emissions (integrated pollution prevention and control) or IED (Industrial Emissions Directive) and provides a framework for legal compliance. Cylinder test gases play a key role in ensuring that the measurements taken by the CEMS are representative and accurate, and therefore compliant with the BS EN 14181 standard. QAL1 is usually done by the CEM supplier, QAL3 is typically undertaken by the operator and QAL2 requires input from a test laboratory – this is where SOCOTEC can step in. Organisations can then be confident that the equipment is tested and reliably reporting on emissions.

QAL1: CEM system installation gases

When a Continuous Emissions Monitor (CEM) is initially installed on site, it needs to be calibrated against a known concentration of test gas.  This is important to ensure the CEM correctly calculates the concentration of test gas, meaning future measurements will be accurate.

Failure to test and suitably calibrate the CEM during installation runs the risk of failing to measure gas correctly. To ensure safe working as well as regulatory compliance, initial calibration is typically completed as per the manufacturer’s instructions as the equipment must be working to the manufacturer’s specification.  When using the test gas, the concentration used would normally relate to the anticipated range of the analyser and there are no requirements for the quality of the gas used at this stage.  

QAL3 checks: drift assessment on-site gases 

Once the CEM is installed and operational, BS EN 14181 requires that regular QAL3 drift assessments are undertaken to establish the continuing precision of the CEMS.

In line with EA guidance, gases traceable to ISO 6141:2000 should be used for this purpose.  To complete the QAL3 check, neither the EA nor EN 14181 guidance outlines suitable concentrations for the gas; however, it is suggested that concentrations of 80 to 100% of the short term ELV would be best.

Functional tests

Under EN 14181, functional tests are required to check the systems operate as per the manufacturers’ specification. The functional test is the first step in both the QAL2 and Annual Surveillance Tests (AST) and includes linearity assessments, zero/span checks, response time testing, documentation audits and system inspections etc.

Linearity assessment gases

Linearity assessments are undertaken using gases at varied concentration levels of 20%, 40%, 60% & 80% of twice the daily ELV. EA guidance acknowledges that there may be some benefit in using the short term ELV instead of twice the daily ELV when selecting suitable concentrations.  It may also be advisable to perform an additional test at 100% to allow maximum possible extensions of the valid range determined during a QAL2 or AST.

If linearity data is to be used in the QAL2 or AST calculations then the gases used must be traceable to EN ISO 17025 to ensure that QAL2 functions can be verified as quality assured.

Linearity data will be used in the calculations where the concentration measured during parallel testing is low, known as a method C approach.  Emissions are defined as low when the spread of collected data pairs is less than the allowable uncertainty for the specific component and the lowest data pair is less than 15% of the daily ELV. Therefore, when emissions are expected to be low, the gases used for linearity checks, along with the gas blender used, must meet the uncertainty requirements laid down.

Test house calibration gases

As part of the QAL2 or AST, parallel testing will take place to evaluate the functionality of the CEM and to ensure the calibration is valid.

Test houses performing QAL2 and AST parallel testing must calibrate their analysers with gases traceable to ISO 17025.  However, under the test house MCERTS accreditation, the test house may be permitted to use transferable gas standards on site, i.e. test gases that are cross-compared with ISO 17025 gases beforehand.  The concentrations used will be determined by the specific method and type of test being performed.


To ensure compliance with BS EN 14181’s QALs, there are typically three gas standards used: unaccredited, ISO 6141 and ISO 17025.  Unaccredited gases should be avoided where possible (not all components may be available to accredited standards).  Site gases would normally be ISO6141 whereas functional test gases and Test house calibration gases are ISO17025.  The concentrations used in each of the areas defined above are likely to be different.

If you would like any assistance in undertaking emissions monitoring in line with BS EN 14181, please get in touch with SOCOTEC to discuss.